1. Field of The Invention
This invention relates to a method of controlling a traffic terminal(s) in a TDMA (Time Division Multiple Access) satellite communications system, and more specifically to such a method which is suited for use with systems wherein more than two reference stations are available for traffic control.
2. Description of the Prior Art
In the TDMA system, each earth station (viz., reference station or traffic terminal) is allowed to transmit bursts (viz., high speed transmission of data bits) in a manner that each burst is located within an allocated time slot of each consecutive TDMA frame. The burst, therefore, has the same period as the TDMA frame and hence plays a vital role in TDMA system synchronization control.
When initially establishing communications between earth stations, burst transmit timing is unknown to a controlled earth station and hence burst acquisition support is necessary prior to the burst synchronization control. Viz., when a reference station intends to initially control a traffic terminal or terminals, the reference station is first required to perform the burst acquisition control with the traffic terminal(s) to be controlled.
More specifically, in such an initial stage between the earth stations, the reference station initially transmits acquisition control information to the controlled terminal, and opens a wide window at a burst acquisition control position (viz., acquisition window) in order to detect the burst which is to be transmitted from the traffic terminal in response to the control information contained in a reference burst. When the burst is received within the acquisition window, the reference station acquires same, measures the received burst position and thereafter computes the difference between the actually received burst position and the nominal position of the traffic terminal. Subsequently, the reference station transmits burst shift information based on the measured position difference.
Thereafter, the reference station closes the above-mentioned window and opens a new window (viz., synchronization window) at the nominal position of the traffic terminal in order to receive a burst which is to transmitted from the controlled (i.e., traffic) terminal If the reference station receives the burst within the new window, it terminates the burst acquisition control to start the next control, viz., synchronization control.
In the case where a single reference station controls one or more traffic terminals, the above-mentioned burst acquisition control does not encounter any problems.
However, in the case where more than two reference stations control the same traffic terminal(s), the above-mentioned system encounters a drawback if there exists a difference between the nominal and acquisition positions of the traffic terminal. FIG. 1 shows a format of an example of the nominal positions of the two (first and second) reference stations and traffic terminal together with the decision (synchronization and acquisition) windows.
More specifically with the above-mentioned drawback, it is practically impossible for the plurality of reference stations to simultaneously control the same traffic terminal(s). Consequently, when a first reference station has completed the burst acquisition control with the traffic terminal(s), the second or later coming reference station is no longer able to receive, at its burst acquisition window, the burst transmitted from the traffic terminal to be controlled. This is because the traffic terminal(s), already controlled by the first reference station, has transmitted its burst at the nominal position thereof. This indicates that the later coming reference station continues (ties up) to transmit the acquisition control information to the traffic terminal(s) while opening the acquisition window and hence is unable to shift the control thereof to the synchronization support.
For further information of the TDMA system, reference should be had to "INTELSAT, TDMA REFERENCE STATION EQUIPMENT SPEC. (Intel 196).